Electric power systems provide an essential service to any modern society. They are inherently large-scale dynamic systems with a high degree of spatio-temporal complexity. Their reliability and security of supply are central considerations in any regional or global energy-related policy. Methods for power systems planning have typically ensured key operational reliability aspects under normal operating conditions and in response to anticipated demand variability, uncertainty and supply disruptions, e.g. due to errors in load forecasts and to unexpected generation units outages. Solutions have been commonly built on capacity adequacy and operating reserves requirements, among others. However, recent objectives for environmental sustainability and the threats of climate change are challenging the reliability requirements of power systems in various new ways and necessitate adapted planning methods. The present chapter describes some of the issues related to the development of the integrated techno-economic modeling and robust optimization framework that is needed today for power systems planning adapted. Such planning framework should cope with the new context by addressing the challenges associated with the sustainability targets of future power systems, and most notably ensuring operational flexibility against the variability of renewable energy sources, ensuring resilience against extreme weather events and ensuring robustness against the uncertainties inherent in both the electric power supply and system load.